All the injured riders were released from the hospital Friday, the day after the Pony Express horseback-style launch coaster failed to make it up an incline and rolled back into the loading station, slamming into another train waiting to depart, said Jennifer Blazey, spokeswoman for the Buena Park theme park.

Ride manufacturer Zamperla, which is based in Italy, released a statement saying it would fully cooperate with the ongoing accident investigation by the California Division of Occupational Safety and Health (DOSH).

A Zamperla representative will visit Knott’s to investigate whether any "programming problems" led to the accident, Blazey said.

Next week, DOSH investigators will look at the mechanics of the ride, attempt to isolate the malfunction, review the ride’s maintenance records and interview ride operators, said Dean Fryer, spokesman for the DOSH amusement ride unit.

Investigators will be looking at whether the Pony Express track featured an anti-rollback mechanism or a zone protection system designed to keep trains running on the same track separated, Fryer said.

The relatively mild coaster, designed for the 'tween set, reaches a height of 44 feet and a speed of 38 miles per hour over an undulating, figure-eight course. Riders straddle the saddle-like coaster seat like a horseback rider. An automated restraint system presses against the rider’s lower back, providing little support for the shoulders and neck.

The ride will remain closed until the state investigation is completed, Blazey said, adding that the train waiting in the station was damaged in the accident.

In September 2009, a cable snapped on the hydraulically launched Xcelerator coaster at Knott's, spraying debris that injured two riders. A state investigation blamed the Xcelerator accident on both Knott's and the ride manufacturer, Switzerland-based Intamin AG.